1. Field of the Invention
The present invention relates to a switching power supply, which can regulate the amount of limitation of power supply output power according to the input or output of the switching power supply and diminish overshoot of inrush current and output voltage during starting.
2. Description of Related Art
Although switching power supplies, which are able to transmit electrical power to a load at an output voltage that differs from the input voltage by performing conventional switching, are becoming the mainstream type of power supply in order to allow reduced size and higher efficiency, since the smoothing circuit has a large capacitance, there has been the problem of device destruction caused by the flow of large inrush current during power supply starting when a charge has not accumulated in the capacitor, as well as the problem of the occurrence of the phenomenon known as overshoot in which output voltage ends up temporarily exceeding the desired voltage.
Therefore, a method referred to as soft starting is employed in which the duty of the signal applied to the switching element during power supply starting is gradually increased to diminish this inrush current and output voltage overshoot, and an example of this prior art is shown in FIG. 2.
The example of the prior art shown in FIG. 2 is a step-down chopper that uses pulse width modulation (to be abbreviated as PWM), and is composed of switching element 1 that generates a pulse voltage by switching the input voltage, smoothing device 2 that smoothes said pulse voltage, detection device 3 that detects power supply output voltage 16, power supply output current 17 or both, switching signal generation device 4 that receives the signal detected by this detection device 3 and generates a drive signal of switching element 1, and soft start circuit 11 that generates a soft start signal.
In addition, in the prior embodiment, switching signal generation device 4 is provided error amplifier 8 that compares power supply output voltage 16 with a reference voltage 14 and transmits a signal in which that error is amplified, and PWM modulator 9 that performs pulse width modulation of the signal transmitted by this error amplifier 8. Moreover, soft start circuit 11 is composed by connecting a constant voltage source like reference voltage 14, resistor 12 and capacitor 13 in series.
The example of the prior art composed in the manner described above operates in the following manner. Power supply output voltage 16 is compared with reference voltage 14, the error amplification signal voltage of those compared voltages is subjected to pulse width modulation by using as the input voltage of PWM modulator 9, switching element 1 is switched according to the output signal of this modulator 9, and the output of this switching element 1 is input to smoothing circuit 2 to be able to obtain power supply output voltage 16.
PWM functions so as to supply large electrical power to the load by increasing the pulse width, namely by increasing the duty, when a large power supply output current 17 is required, and reduce electrical power to the load by decreasing the duty when power supply output current 17 is small.
PWM modulator 9 that performs PWM is composed of a comparator having three inputs for a chopping wave that serves as a reference, an error amplification signal voltage and a soft start signal, and realizes PWM was realized by comparing the signal of the lower voltage valve between the error amplification signal voltage and soft start signal with the chopping wave.
This example of the prior art is equipped with a typical soft start circuit 11, accumulation of charge in the above capacitor begins from the time the power supply is started, and a soft start signal is generated by gradually increasing the voltage of the terminals connecting the above resistor 12 and the above capacitor 13 over an amount of time determined by the product of the value of the above resistor 12 and the value of the above capacitor 13.
However, since the above capacitor 13 typically required a large capacitance on the order of 100 xcexcF in order to suppress inrush current without allowing overshoot, integration was difficult and there were problems with respect to an increase in the number of parts and impairment of power supply size reduction.
Moreover, in the case of, for example, controlling the power supply by a modulation system different from PWM, there was the problem of being unable to use a method that uses the above three-input comparator.
That is, since the method of the prior art required a large capacitance to generate a soft start signal, it was difficult to reduce the size of the power supply and attain a higher degree of integration, and there was also the problem of not being able to conform to certain power supplies depending on the modulation system used to control the power supply.
In consideration of the above problems, the object of the invention is to provide a switching power supply that is capable of limiting the power supply output power using a method that is compatible with compact size regardless of the modulation system.
In order to achieve the above object, the invention according to a first aspect of the present invention relates to a switching power supply that is capable of regulating the limit of power supply output power as desired by applying a pulse signal to a switching element that forcibly switches off that switching element according to a signal obtained by detecting the input voltage, output voltage, input current or output current of the power supply.
The switching power supply according to the present invention is provided with a switching element that converts an input voltage to a pulse voltage, the output of this switching element is provided with a smoothing device for smoothing the above pulse voltage, and the output of this smoothing device is composed so as to be connected to a load in the form of the power supply output.
Moreover, the above switching power supply is provided with a detection device that detects the power supply output voltage, power supply output current or both, and is equipped with a switching signal generation device for generating a switching element drive signal according to the detected signal.
The output of the above switching signal generation device is connected to one input of a reset device that is able to switch off the switching element regardless of the signal of the switching signal generation device, and is connected to the switching element.
The other input of the above reset device is connected to the output of a pulse signal generation device that receives a signal from a control signal detection device that detects at least one of input voltage, input current, output voltage or output current, and generates a signal for reset, and is capable of regulating the limit of power supply output power by changing the duty of the above pulse signal.
In addition, the power supply output power limit can also be regulated by inputting a signal that has detected either power supply internal voltage or power supply internal current in the form of a power supply internal signal such as a reference voltage, in addition to the above detection signal, to the above reset device input.
FIG. 3 indicates the relationship among the drive signal of a switching element, pulse signal for resetting, and reset device output signal, and the switching element drive signal drives the switching element as is when the reset pulse signal passes through, while a signal that switches off the switching element is applied to the switching element when the reset pulse signal is reset.
As a result, the maximum value of the duty of the reset device output signal is determined by the duty of the reset pulse regardless of the switching element drive signal, and since that maximum value cannot be equal to or greater than the duty of the reset pulse, the power supply output power can be limited.
In addition, the magnitude of power limitation can be regulated as desired by changing the duty of the reset pulse between 0% and 100%.
For example, soft starting of the power supply can be realized by gradually changing the duty of the reset pulse together with starting of the power supply, and gradually decreasing the amount of power limitation.
Although a pulse signal generation device for realizing the above soft start can be realized by using a resistor and capacitor similar to those of conventional methods, differing from conventional methods, it can also be realized with a simple logic circuit without using a capacitor, thereby enabling a higher level of integration and compact size.
Moreover, since the reset device is provided after the switching signal generation device in the present invention, limitation of output power can be realized regardless of the modulation system within the switching signal generation device.
That is, the present invention allows power supply output power to be regulated regardless of the modulation system used for power supply control by changing the duty of the reset pulse signal, thereby making it possible to provide a compact switching power supply free of inrush current and overshoot.
According to the first aspect of the present invention, the amount of limitation of power supply output power can be regulated by changing the duty of a reset pulse signal applied to a reset device that resets the switching element drive signal, thereby making it possible to provide a compact switching power supply that is free of inrush current and overshoot during starting in the case of adapting to soft starting during power supply starting.
According to the second aspect of the present invention, a switching power supply can be provided that allows power supply output power to be regulated by various modulation systems without being affected by the type of modulation system by providing the reset device after the switching signal generation device output.
According to the invention according to claim 3, a switching power supply can be provided that enables the amount of limitation of power supply output power to be changed gradually in the manner of, for example, a soft start circuit.